Conventional selectorized weight machines utilize a stack of individual weight plates that move vertically on a pair of parallel bars. These weights are typically coupled to the resistive mechanism of the machine by cables or belts traveling over pulleys and a variable resistance cam. They may also be directly linked by a lever system such as is described in U.S. Letters Pat. No. RE31113 entitled Variable Resistance Lifting Device. The variable resistance generated by the cam in these machines conforms to a biomechanical pattern resulting in a uniform feeling exercise pattern, conforming to the desires and expectations of most users.
Typically, prior art machines, such as those designed for leg extensions, leg curls and arm curls, have to precisely align the user's joint center with the machine axis. This has been critical for the proper function and feel of the machine. Without this proper alignment, as the user moves through the motion arc of the machine, the user and the machine move through different arcs, causing the user to bind or reach during the motion.
These prior art machines all have the additional disadvantage of a large number of components which are subject to wear, and a significant vertical clearance required to house the moveable weight stack.
Free-weight machines that utilize individual weights (placed on the machine by the user) have now been designed to approximate the exercise profile of the selectorized weight machines. Unfortunately, each weight plate has to be loaded and unloaded by hand, causing inconvenience to the user and added cost to the owner of the exercise facility, as large quantities and combinations of weights must be present to satisfy the needs of different users. Additionally, the levers and linkages installed in these free-weight machines to approximate the selectorized weight machine profile are often inaccurate, and the user may experience an undesirable resistance profile.
Electronic means have been employed to vary the position of a constant mass or weight along the lever. Patents describing such devices include the Telle (U.S. Pat. No. 4,863,161), Cartwright (U.S. Pat. No. 4,650,185) and Jungeris (U.S. Pat. No. 3,588,101). Unfortunately, electric motors result in increased cost and maintenance.
What is needed is an exercise device in which resistance can be easily adjusted by the user, that will match a predetermined resistance profile throughout the exercise stroke (i.e., the user starting in an at-rest position, exerting force against resistance generated by the exercise device and then reversing the process), does not require a high vertical clearance for a weight stack, and is low in maintenance.